The present invention relates to an optical amplifier used for optical communication systems, and particularly to an optical amplifier used suitably for systems which adopt the optical transmission scheme based on wavelength division multiplexing.
Recently, optical transmission systems which use optical amplifiers as repeaters are studied and put into practice vigorously. It is particularly beneficial for the multi-media service industry typified by the Internet to increase the band width and communication capacity based on the WDM (Wavelength Division Multiplex) technology for multiplexing signal lights of different wavelengths. In the optical communication system using the WDM technique, the optical repeater which amplifies all signal lights at once has an extremely crucial role for the extension of transmission distance.
For the optical amplification medium which forms the optical amplifier, optical fiber, with rare earth being doped thereto, is useful, and it is under study and about to be put into practice. Particularly, erbium-doped fiber (EDF), which is active for amplification in a wide range of wavelength in which the loss of optical fiber is small, is used extensively in optical fiber communication systems.
In order for the optical amplification medium to be active for amplification in the band of signal light wavelengths, a pumping light which is shorter in wavelength than the signal light is put together with the signal light into the medium. The optical amplification medium has at its end the connection of a WDM optical coupler so that the signal light and pumping light are incident efficiently to the medium.
In case that a number of optical amplifiers are used as repeaters in an optical fiber communication system, it is desirable from the viewpoint of transmission design to have a constant output light power of the signal light immediately after each optical amplifier. The simplest known technique is automatic level control (ALC), which measures the total output light power of WDM signal light of the optical amplification medium and adjusts the pumping light power so that the measured power is constant.
However, the rare-earth-doped optical fiber is narrow in its gain flatness region against signal wavelengths in amplifying the signal lights at once, and even with the imposition of limitation of the wavelength range, the gain flatness against signal wavelengths is deteriorated by the variation of input signal light power.
As a scheme of overcoming this problem, there is known a technique for suppressing the wavelength dependency of the gain at different input light power levels, in which the input light power and output light power of the optical amplification medium are measured and the pumping light power is adjusted based on automatic gain control (AGC) so that the average gain evaluated from the measured light power levels is constant. The optical amplifier based on this scheme further adopts constant output control by use of a variable optical attenuator.
However, the actual optical amplifier has its gain characteristics varied by the temperature variation in addition to the variation of input light power. Therefore, an optical amplifier, which is designed to have a minimal gain""s wavelength dependency at a certain temperature, cannot be rid of the wavelength dependency based solely on the AGC due to the variation of operating temperature.
Namely, the actual optical amplifier has its gain characteristics varied in response to the variation of input light power and temperature, and therefore it is necessary to provide an additional function of suppressing the wavelength dependency by monitoring the gain or output light power of each signal light.
With the intention of solving this problem, there are known several techniques for flattening the gain of optical amplifier throughout the wavelengths. For example, a control technique disclosed in Japanese Patent Laid-open (kokai) No. Hei 10-303821 is designed to separate the wavelength-multiplexed signal lights by using an array waveguide grating (AWG) and monitor the power of individual signal lights with arrayed photodiodes. Another control technique disclosed in Japanese Patent Laid-open (Kokai) No. Hei 10-173266 is designed to separate part of the output light of optical amplification medium and monitor the power of each signal light based on scanning with a variable wavelength band-pass filter thereby to compensate the gain""s wavelength dependency.
However, the AWG and variable wavelength band-pass filter for separating the wavelength-multiplexed signal lights are expensive optical parts, and therefore the optical amplifier using any of these parts is inevitably expensive.
With the intention of overcoming the above-mentioned prior art problem, a first object of the present invention is to provide a simple optical amplifier and an optical amplification method capable of evaluating the gain characteristics in the presence of the variation of input light power and temperature.
A second object of the present invention is to provide an optical amplifier and an optical amplification method capable of attaining the intended gain tilt based on the evaluated gain tilt.
The inventors of the present invention have conducted experiments on rare-earth-doped fiber, and found that the output light power to pumping light power relation plotted as proper curves for various input light power levels and temperatures, with the gain""s wavelength dependency being kept constant, i.e., the gain tilt is constant throughout the wavelengths, are simple increase functions or linear functions.
Accordingly, by recording proper curves for several gain tilts in advance, and, in operation of the optical amplifier, selecting a proper curve on which the measured output light power to pumping light power relation exists, it is possible to determine the gain tilt corresponding to the selected proper curve as the gain tilt of the optical amplifier in operation. If there is no proper curve on which the output light power to pumping light power relation exists, the proper curve on which the relation nearest the output light power to the pumping light power relation exists is selected, and the gain tilt corresponding to selected characteristic curve is adopted as the gain tilt of the optical amplifier.
For more accurate determination of gain tilt in the absence of the exact characteristic curve, a first proper curve on which the relation nearest the output light power to pumping light power relation exists and a second proper curve on which the relation next nearest the output light power to pumping light power relation exists are selected and a third proper curve located between the first and second proper curves is set based on the interpolation, and the gain tilt corresponding to the third proper curve is determined as the gain tilt of the optical amplifier.
Among the affairs of the present invention disclosed in this specification, representatives are summarized as follows.
(1) An optical amplifier comprises an optical amplification medium which has simultaneous inputs of a WDM signal light and a pumping light and amplifies the signal light, a light source which generates the pumping light, and an amplification characteristic supervisor part which evaluates the gain tilt of amplified signal lights of different wavelengths which form the WDM signal light. The amplification characteristic supervisor part is provided in advance with a record of a plurality of proper curves formed by plotting, for certain gain tilts, the relation between the optical amplification medium output light power and the pumping light power corresponding to the output light power. And, the amplification supervisor part selects, from among the recorded proper curves, a proper curve A which is nearest to the relation in operation of the power of the pumping light which is incident to the optical amplification medium in operation and the resulting amplified output light power, thereby determining the gain tilt of the optical amplification medium based on the selected proper curve A.
(2) More preferably, if the relation in operation exists just on the proper curve A, the amplification characteristic supervisor part determines the gain tilt of the proper curve A to be the gain tilt of the optical amplification medium, or otherwise if the relation in operation exists out of the proper curve A, it selects another proper curve B which is next nearest to the relation in operation and sets a proper curve C located between the proper curves A and B based on the interpolation, thereby determining the gain tilt corresponding to the proper curve C as the gain tilt of the optical amplification medium.
(3) An optical amplifier comprises an optical amplification medium which has simultaneous inputs of a WDM signal light and a pumping light and amplifies the signal light, a light source which generates the pumping light, an amplification characteristic supervisor part which evaluates the gain tilt of amplified signal lights of different wavelengths which form the WDM signal light, and a compensation part of amplification characteristic which controls the gains of individual wavelengths by using the difference between an intended gain tilt and the gain tilt evaluated by the amplification characteristic supervisor part so that the optical amplifier output light has the intended gain tilt. The amplification characteristic supervisor part is provided in advance with a record of a plurality of proper curves formed by-plotting, for certain gain tilts, the relation between the optical amplification medium output light power and the pumping light power corresponding to the output light power. And, the amplification supervisor part selects, from among the recorded proper curves, a proper curve A which is nearest to the relation in operation in operation of the power of the pumping light which is incident to the optical amplification medium in operation and the resulting amplified output light power exists, thereby determining the gain tilt of the optical amplification medium based on the selected proper curve A.
The optical amplifiers of items (1) and (2) may have their several proper curves recorded in the form of characteristic data tables in a semiconductor ROM (read only memory), and their selection of proper curve A and following determination of gain tilt accomplished by an ordinary semiconductor processor, instead of needing expensive optical parts. Consequently, the gain characteristics (gain tilt) in the presence of the variation of input light power and temperature can be evaluated by a simple arrangement without using expensive component parts, and the first object of the present invention is attained.
The optical amplifier of item (3) can get the gain tilt of the optical amplifier in operation to evaluate the difference between the intended gain tilt and the gain tilt of the optical amplifier in operation, and uses their difference for controlling the gain tilt as intended, and the second object of the present invention is attained.
These and other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.